1. Field of the Invention
The present invention relates to a navigation system using an angular rate sensor, and more specifically to a navigation system by which automotive vehicle travel locations are detected by an angular rate sensor and a distance sensor and the detected vehicle locations are projected on a displayed map road, so that the vehicle travel display moves along a predetermined route on the map.
Since this system can operate independently without the need of other auxiliary means (e.g. radio signals), this system is effective in an an area where radio navigation systems cannot operate reliably, such as an urban area or other areas subject to radio interference.
2. Description of the Prior Art
The applicant has already proposed a dead reckoning and map correlation system for automotive vehicle tracking, which uses an angular rate sensor and a distance sensor, in Japanese Unexamined Published (Kokai) Patent Application No. 60-48600, entitled Vehicle Position Detecting System. In this system, current vehicle locations can be intermittently detected on the basis of vehicle travel distance data and vehicle travel angle data ((detected by an angular rate (velocity) sensor)); and the calculated vehicle locations are projected onto the roads displayed on a digital map (including road intersections and curves) previously prepared by inputting digital map data to a CPU through a keyboard so that the vehicle travel motion can track a road displayed on the map.
In the navigation system of this kind, it is extremely difficult to correct the error between the current location and the map road, so that the displayed vehicle motion can correctly track a predetermined road on the digital map. The correction method of the above-mentioned prior-art system will be described with reference the attached drawings. In FIG. 1(A), P'(X, Y) denotes coordinates of the current detected vehicle location point (not displayed); P.sub.1 (X.sub.1, Y.sub.1) denotes coordinates of an intersection of roads; S.sub.1 denotes a distance between the detected vehicle location data P' and the intersection data P.sub.1 ; .theta. denotes an angle data subtended by the two lines P.sub.1 P' and P.sub.1 P.sub.01. Further, the dashed curve denotes an actual vehicle travel route (not displayed).
The above distance data S.sub.1 and the angle data .theta. are detected for each predetermined distance to obtain the current detected vehicle location P'(X, Y). In FIG. 1(A), since the calculated position P' deviates from the route R, this position P' is projected on the route R for correction. That is, a corrected vehicle location P.sub.01 (X.sub.01, Y.sub.01) is calculated in accordance with the following expression: EQU X.sub.01 =X.sub.1 +S.sub.1 cos .theta. EQU Y.sub.01 =Y.sub.1 +S.sub.1 sin .theta. EQU S.sub.1 =.sqroot.(X.sub.1 -X).sup.2 +(Y.sub.1 +Y).sup.2
The above-mentioned correction method is fairly effective when the vehicle travels along a straight road. However, when the vehicle turns along an intersection or a curved road, various problems arise as follows:
(1) 1st problem
In this method, it is possible to reduce a correction error at an intersection as shown in FIG. 1(B). In more detail, even if the vehicle turns at an intersection as shown by dashed lines, since the distance D.sub.e can automatically be corrected at the intersection, the correction error between the actual and corrected points is small.
However, when the vehicle travels along an inflection as shown by dashed lines in FIG. 1(C), since D.sub.e cannot be corrected, there exists a large error e.
(2) 2nd problem
In this system, a route judge area as shown in FIG. 2(A) is determined at each branch point P.sub.1 to determine a turning point for the vehicle. That is, when a distance L.sub.1 between the current vehicle location P.sub.00 and the succeeding branch point P.sub.1 becomes less than a predetermined value L.sub.1, since this indicates that the vehicle approaches a branch point P.sub.1, a revised distance calculation begins to detect that the vehicle has passed through the point P.sub.1. And, when the calculated distance exceeds a predetermined value (L.sub.1 +L.sub.2), it is determined that the vehicle has turned at the branch point P.sub.1 by detecting the travel angle.
In the prior-art method, however, since this route judge area (L.sub.1 +L.sub.2) is fixed, when an adjacent point P.sub.2 is close to the point P.sub.1, there exists a problem in that the vehicle passes beyond the adjacent point P.sub.2 before the vehicle turns at the point P.sub.1, thus resulting in route change judgement error. Further, there exists another problem in that it is impossible to determine a sufficient route judgement distance between the two points P.sub.1 and P.sub.2.
Further, there exists another problem in that when a distance between two points P.sub.1 and P.sub.s is long and a large error occurs, the calculated distance to the point is quite different from an actual distance, so that it is impossible to detect a route judge area when the vehicle has passed through the point. The above-mentioned error between the calculated map distance and the actual distance is produced when a curved road R is approximated by a straight line P.sub.1 P.sub.2 as shown in FIG. 2(B). That is, the curved actual distance is larger than the straight map distance when the vehicle travels from P.sub.1 to P.sub.2, and shorter than the straight map distance when the vehicle travels from P.sub.2 to P.sub.1. Therefore, where these road inflections continue as shown in FIG. 2(C), there exists a problem such that cumulative errors are produced.
When the above error is produced, since there exists a difference in distance between the map branch point and the actual branch point, it is impossible to detect route change data within a route judge area JE.
FIG. 2(D) shows an example where the vehicle turns at the actual branch point B.sub.A before the route judge area JE of the map branch point B.sub.M is detected, because the distance between two map branch points is longer than the actual distance. FIG. 2(E) shows an example where the vehicle turns at the actual branch point B.sub.A after the vehicle has passed through the route judge area JE of the map branch point B.sub.M, because the distance between two map branch points is shorter than the actual distance.
(3) 3rd problem
In this system, straight routes between two points (e.g. intersections, branch points, turning corners, etc.) are calculated on the basis of map data including coordinates of these points and point numbers adjacent to each point, and the current vehicle travel locations are displayed along these calculated straight routes. Further, in order to judge a change in vehicle travel direction, a route judge area JE is provided at each point P.sub.1 as shown in FIGS. 3(A) and 3(B).
The third problem is how to display the current vehicle location within this route judge area JE.
In the prior-art system, the current vehicle location is continuously displayed along the route on which the vehicle runs toward the point P.sub.1, as shown by white dots in FIG. 3(A) or fixedly displayed at the point P.sub.1 as shown by a black dot in FIG. 3(B).
Therefore, in the case shown in FIG. 3(A), since the vehicle location is displayed even on an extension of the route P.sub.0 P.sub.1, there exists a problem in that a location mark is dislocated from the route when the vehicle turns at a T-shaped crossing or displayed as if the vehicle travels straight in spite of the fact that the vehicle turns right or left at a crossroads, thus resulting in a display error.
Further, in the case shown in FIG. 3(B), since the display mark jumps from the point P.sub.1 to a corrected location after a route change has been determined, there exists another problem in that the location is displayed in an unnaturally appearing manner.